This invention relates to preparing a plate rim for joining with a wall of an abutting member. More particularly, it relates to preparation for brazing of an outer rim of an end plate or tip cap to an inner wall of a hollow member, for example an inner wall of the tip portion of an air cooled turbomachinery blade.
During one form of the manufacture of a turbine engine high temperature operating, airfoil shaped, generally hollow air-cooled turbine blading member such as a blade or vane, an end plate is bonded at the radially outer portion of the member. Such an end plate, generally referred to in the turbine engine art as a tip cap, has been described in a variety of forms. Typical U.S. Patents discussing some of such forms include U.S. Pat. No. 4,169,020xe2x80x94Stalker et al. (patented Sep. 25, 1979); U.S. Pat. No. 4,214,355xe2x80x94Zelahy (patented Jul. 29, 1980); and U.S. Pat. No. 5,672,261xe2x80x94Wheat et al. (patented Sep. 30, 1997). Of more particular interest in connection with an embodiment of the present invention is the Wheat et al. patent. That patent describes electrochemical removal of material from the inner wall of the member and cleaning products of casting at least from the plate rim such as by at least one of chemical (acid) treatment and a mechanical abrasion type of method, typically tumbling in a abrasive medium. Then a Ni electroplate was provided on the rim to enhance bonding, for example brazing, between the plate rim and an inner wall of a member.
A form of such tip caps described by Wheat et al. is manufactured by typical, well known ceramic mold precision casting of a Ni base superalloy, for example from commercially available Rene"" 80 alloy, forms of which are more fully described in U.S. Pat. No. 3,615,376xe2x80x94Ross (patented Oct. 26, 1971). A form of Rene"" 80 Ni base superalloy generally includes nominally by weight about 9.5% Co, 3% Al, 14% Cr, 5% Ti, 4% W, 4% Mo, 0.17% C, 0.015% B, 0.03% Zr, with the balance Ni and incidental impurities. The as-cast tip cap generally includes on its surface portion oxides, as the product of precision casting ceramics and oxidation of the tip cap alloy surface, as well as the alloy elements themselves, including particularly Al and Ti. All of such products and elements can inhibit the surface wetting of a typical brazing alloy, if one subsequently is used to bond a surface of the tip cap to an airfoil wall.
The method described by the Wheat et al. patent represents one approach to enhancing brazing of the plate rim to an inner wall of a member using such cleaning and a Ni intermediate portion. However, it has been recognized that such a multiplicity of preparation steps and difficulties observed in providing an adequate Ni electroplate on the tip cap rim decreased manufacturing yield and quality, and therefore increased manufacturing cost.
The present invention, in one form, provides, in a method for making an article such as a turbomachinery blading member, the preparation for brazing together first and second substantially uncoated substrate surfaces, for example an end plate rim and an inner surface of a wall of a hollow body open end. The method eliminates use of Ni, for example Ni electroplate, as an intermediate material to enhance wetting by the braze alloy of at least one substrate surface, for example the end plate rim. The substrate alloy surfaces are made of an alloy based on at least one of Fe, Co, and Ni including greater than about 5 weight % total of Al and Ti. The method comprises preparing at least one of the surfaces, for example the end plate rim, by treating the surface with a reducing gas comprising a halogen gas, for example about 3-20 volume % of halogen gas. Treatment is at a temperature and for a time, for example for about 1-6 hours at about 1650-1950xc2x0 F., sufficient for the gas to react with Al and/or Ti in the alloy surface to a depth that avoids intergranular attack (IGA) of the substrate alloy surface, preferably to a depth no greater than about 0.0015xe2x80x3. This converts such elements only in the surface to such depth to a gaseous halide form and depletes such elements to a total level below about 1 wt %. Then the surface or surfaces thus prepared is brazed with an inner surface of the wall.